Lecture 8. Fundamentals of atmospheric chemistry: Part 3. Basic gas-phase chemical reactions in the troposphere and in the stratosphere.

Objectives: 1. Atmospheric species of interest. 2. Summary of basic gas-phase chemical reactions in the troposphere. 3. Summary of basic gas-phase chemical reactions in the stratosphere.

Readings: Turco: p.77-83 ; Brimblecombe: p. 49-54

1. Atmospheric species of interest.

Table 8.1 below lists some inorganic and organic gases that are important in the atmosphere.

Inorganic gases in the table are all those that contain either oxygen, nitrogen, sulfur, chlorine, bromine, and possibly either hydrogen or carbon, but not both hydrogen and carbon.

Organic gases are those that contain both hydrogen and carbon, but may also contain other atoms.

Hydrocarbons are organic gases that contains only hydrogen and carbon. Hydrocarbons are divided primarily into alkanes, cycloalkanes, alkenes, alkynes, aromatics, and terpenes.

Non-methane hydrocarbons (NMHCs) are all hydrocarbons excluding methane. Oxygenated hydrocarbons are hydrocarbons plus oxygenated functional groups, such as aldehydes, ketones, alcohols, acids, and nitrates.

1 Reactive organic gases (ROG) or volatile organic carbon (VOC) are non- methane hydrocarbons plus oxygenated hydrocarbons.

Non-methane organic carbon (NMOC) is non-methane hydrocarbons plus carbonyls (aldehydes and ketones).

Total organic gases (TOG) are methane plus ROGs or VOGs.

• Because the number of organic gases is VERY large, individual organics are often lumped into surrogate carbon bond groups, and the groups, instead of the gases, are used in chemical reactions.

Some examples of grouping: Alkanes are those hydrocarbon molecules in which all the carbon bonds are shared with hydrogen atoms except for minimum number required for carbon-carbon bonds.

For instance, ethane CH3-CH3 Cycloalkanes are alkanes which have a ring structure. NOTE: Alkanes generally react by replacement of a hydrogen atom. Once a hydrogen atom is removed from an alkane, the involved carbon atom has an unpaired electron and the molecule becomes a free radical , in this case an alkyl radical (often denoted as R. ) . For instance, ethyl CH3-CH2 Alkenes are those hydrocarbon molecules in which two neighboring carbon atoms share a pair of electrons, a so-called double bond.

For instance, ethene CH2=CH2 Alkynes are those hydrocarbon molecules in which have triple bond. For instance, acetylene HC---CH

2 Table 8.1 Some atmospheric inorganic and organic gases. For convenience, organic sulfur, chlorine, and bromine-containing species are listed with inorganic species.

Chemical Chemical name Chemical Chemical name formula formula INORGANICS

Oxygen

O(1D) atomic oxygen O2 molecular oxygen (singlet)

O or O(3P) atomic oxygen O3 ozone (triplet) Hydrogen H atomic hydrogen H2 molecular hydrogen Oxygen / Hydrogen . . OH hydroxyl radical HO2 hydroperoxy radical H2O water vapor H2O2 hydrogen peroxide Nitrogen/Oxygen/Hydrogen . N2 molecular nitrogen NO3 nitrate radical NO nitric oxide N2O5 dinitrogen pentoxide NO2 nitrogen dioxide N2O nitrous oxide HONO (HNO2) nitrous acid HO2NO2 peroxynitric acid HNO3 nitric acid Sulfur/Oxygen/Hydrogen/Carbon S atomic sulfur OCS carbonyl sulfide SO sulfur oxide CS carbon monosulfide

SO2 sulfur dioxide CS2 carbon disulfide SO3 sulfur trioxide CH3SH methyl sulfide . HSO3 bisulfite CH3S methyl sulfide radical . HS hydrogen sulfide CH3SCH3 dimethyl sulfide radical (DMS) . H2S hydrogen sulfide CH3SCH2 dimethyl sulfide radical H2SO4 sulfuric acid CH3SSCH3 methyl disulfide methane sulfonic acid MSA CH3SO3H

3 Chlorine/Oxygen/Hydrogen/ Nitrogen /Fluorine/Carbon HCl hydrochloric acid Cl2O2 dichlorine dioxide Cl atomic chlorine CH3Cl methyl chloride ClO chlorine monoxide CH3CCl3 methyl chloroform . chlorine peroxy radical OClO CCl4 carbon tetrachloride . chlorine peroxy radical trichlorofluoromethane ClOO CFCl3 (CFC-11) dichlorofluoromethane HOCl hydrochlorous acid CF2Cl2 (CFC-12) 1-fluorodichloro, 2- ClONO2 chlorine nitrate CFCl2CF2Cl difluorochloroethane (CFC-113) chlorodifluoromethane ClNO2 chlorine nitrite CF2ClH (HCFC-22)

Cl2 chlorine gas Bromine/Oxygen/Hydrogen/ Nitrogen /Carbon HBr hydrobromic acid BrONO2 bromine nitrate Br atomic bromine CH2Br methyl bromide BrO bromine monoxide Br2 molecular bromine HOBr hypobromous acid BrCl bromine chloride Carbon/Oxygen CO carbon monoxide CO2 carbon dioxide ORGANICS

Alkanes (single bond) CH4 methane CH3(CH2)5CH3 heptane CH3CH3 ethane CH3(CH2)6CH3 octane CH3CH2CH3 propane CH3(CH2)7CH3 nonane CH3(CH2)2CH3 n-butane CH3(CH2)13CH3 pentadecane CH3(CH2)3CH3 pentane C4H10 2-methylpropane CH3(CH2)4CH3 hexane C4H9C(CH3)3 2,2-dmethyl hexane Cyclo Alkanes CH2CH2CH2 cyclopropane CH3C5H9 methylcyclopentane CH2(CH2CH2)2 cyclopentane CH3C6H11 methylcyclohexane Alkenes (double bond) CH2CH2 ethene (ethylene) CH3CHCHCH3 trans 2-butene CH2CHCH2 propene (propylene) CH3CH2CHCH2 1-butene Cyclo Alkenes C5H8 cyclopentene C6H10 cyclohexene

4 Alkynes (triple bond) C2H2 acetylene Aromatics (double bond, with a ‘benzene’ ring structure) C6H6 benzene C6H5CH3 toluene C6H5C2H5 ethylbenxene C6H4(CH3)2 o-xylene 1,2,3-trimethylbenzene (CH3)C6H3 C6H4(CH3)2 m-xylene Terpenes α C5H8 isoprene C10H16 -pinene Aldehydes (double bond between carbon and oxygen) CH2O formaldehyde CH3CH2CHO propionaldehyde CH3CHO acetaldehyde C6H5CHO benzaldehyde HOCH2CHO glycol aldehyde Ketones (double bond between carbon and oxygen) CH3C(O)CH3 acetone CH3CH2COCH3 methylethylketone Alcohols CH3OH methanol CH3CH2OH ethanol (methyl alcohol)

C6H5OH phenol Carboxylic Acid HC(O)OH formic acid CH3C(O)OH acetic acid Nitrates/Nitrites/Nitric Acids CH3ONO2 methyl nitrate CH3C(O)OONO2 peroxyacyl nitrate (PAN) CH3ONO methyl nitrite CH3O2NO2 methylperoxy nitric acid C H (CH )(OH)NO C6H5CH3ONO2 benzyl nitrate 6 3 3 2 nitrocresol Alkyl Radicals . . CH3 methyl radical CH3CH2 ethyl radical Alkoxy Radicals . . CH3O methoxy radical CH3CH2O ethoxy radical Alkylperoxy Radicals . methyperoxy radical . CH3O2 CH3CH2O2 ethylperoxy radical Acyl Radicals . . HCO formyl radical CH3CO acyl radical Acyloxy Radicals . CH3C(O)O acyloxy radical Acylperoxy Radicals . . CH3C(O)OO peroxyacyl radical CH2CCH3C(O)OO methyl peroxyacyl radical

5 2. Summary of basic gas-phase chemical reactions in the troposphere.

The troposphere behaves as a chemical reservoir distinct from the stratosphere. Some differences important for chemistry between the troposphere and the stratosphere are: 1. In general , the troposphere has warmer temperature and higher pressure; and higher concentration of atmospheric gases (including water vapor); 2. Less UV radiation is reaching the troposphere; 3. In general, clouds and aerosols are more abundant in the troposhere; Therefore, it is common to distinguish between the chemistry of the troposphere and the chemistry of the stratosphere. In turn, the chemistry of the troposphere depends on tropospheric conditions: such as clean (background) troposphere or polluted troposphere.

NOTE: the chemistry of the stratosphere focuses on the reactions that form and destroy ozone (it will be discussed in Lecture 35-38)

The gas-phase chemistry of the troposphere:

• The troposphere is an oxidative medium, the tendency is for species to be moved to a more oxidized state. For instance,

Hydrocarbons
oxygenated hydrocarbons
acids
CO2;

Reduced sulfur compounds (e.g., H2S, CH3SCH3)
SO2
H2SO4;

NO
NO2
HNO3; • The oxidation of organic species proceeds in the presence of oxides of nitrogen under the action of light. The predominant hydrocarbon in the troposphere is methane.

6 • In simple term one can say that hydroxyl radical controls daytime chemistry and the nitrate radical controls chemistry at night.

Basic photochemical cycle of NOx (NO2+NO), and O3.

(1) NO2 + hν
NO + O

NO2 absorbs at wavelengths < 424 nm

(2) O + O2 + M
O3 + M

M represents O2 or N2 that absorbs the excess of energy

NOTE: This reaction is the main source of O3 (3) NO + O3
NO2 + O2

(1), (2) and (3) are strongly coupled. If J1, k2, and k3 are the reaction rate coefficients, then the rate equation for NO2 is :

d[NO2] / dt = k3 [NO] [O3] - J1[NO2]

If this equation is considered steady-state, then it set to zero and the concentration of O3 can be determined

[O3] = J1[NO2] / ( k3 [NO]) which is known as photostationary state relationship.

However, the mixing ratio of O3 measured in urban and regional troposhere are often greater than those calculated from the photostationary state relationship. It implies that other reactions than (1)-(3) are important.

Moreover, in the troposhere O3 is detroyed by photolysis 1 O3 + hν
O2 + O( D) (for wavelengths < 310 nm)

O3 + hν
O2 + O (for wavelengths > 310 nm)

7 Produced excited atomic oxygen can react with water vapor to form the hydroxyl radical: 1 . O( D) + H2O
2 OH

• The hydroxyl radical is extremely important in the tropospheric chemistry. It converts

NO2 to HNO3 , oxidizes carbon monoxide and reduced sulfur compounds, etc.

Basic chemistry of carbon monoxide and NOx.

Oxidation of CO results in additional O3 production. It can be summarized as follows: . . CO + OH
CO2 + HO2 (with O2 present) . . HO2 + NO
NO2 + OH

NO2 + hν
NO + O

O + O2 + M
O3 + M

Net: CO + 2O2 + hν
CO2 + O3

Note: that net formation of O3 occurs because the conversion of NO to NO2 is accomplished by the HO2 radical rather then by O3 itself.

Note: neither OH nor HO2 radical is consumed in this reaction, which can be viewed as a catalytic oxidation of CO to CO2

• Catalysts are substances which alter the rate of a chemical reaction without themselves appearing in the end products. Catalysts act by changing the mechanism of a reaction, so to allow progression via a transition state of lower energy, hence causing a lowering of the activation energy of the reaction.

8 Note: the basic reaction mechanism of the CO/NOx system exhibits many of the key features of those involving complex organic molecules. In particular, the role of OH as the oxidizing species and the NO to NO2 conversion by HO2 are central to virtually every atmospheric organic/NOx mechanism.

Basic chemistry of methane.

Oxidation of CH4 (which the most abundant organic gas) results in additional O3 production. It can be summarized as follows: . . CH4 + OH
CH3O2 + H2O (with O2 present) . . CH3O2 + NO
CH3O + NO2 . . CH3O +O2
HCHO + HO2 . . HO2 + NO
OH + NO2

2 (NO2 + hν
NO + O )

2 (O + O2 + M
O3 + M)

Net: CH4 + 4O2 + 2 hν
HCHO + 2 O3 + H2O

Basic chemistry of sulfur. Sulfur oxides: 2 SO2 + O2
2 SO3 this reaction is very slow under catalyst-free conditions in the gas phase, that it can be totally neglected as a source of SO3

Dominant reaction for SO2: . . SO2 + OH + M
HOSO2 + M . . HOSO2 + O2
2 HO2 + SO3 in the presence of water vapor

SO3 + H2O + M
H2SO4 + M

9 Reduced sulfur compounds (such as H2S, CH3SH, CH3SCH3, CS2, . . OCS): react with OH and NO3

The DMS-OH reaction proceeds via H-atom abstraction or OH addition to the sulfur atom in the DMS molecule: . . CH3SCH3+ OH
CH3SCH2 + H2O . . CH3SCH3+ OH 
CH3S(OH)CH3 then a complex chain of reactions take place leading to production of SO2
SO3
H2SO4

The DMS-NO3 reaction proceeds initially via H-atom abstraction: . CH3SCH3+ NO3
CH3SCH2 + H2NO3

NOTE: the DMS oxidation chemistry is very complex, and is not completely understood at the moment

Hydroxyl radical , OH. is extremely important in day-time atmospheric chemistry.

There are many formation mechanisms for OH. such as:

. HNO2 + hν
OH + NO

. H2O2 + hν
2 OH

1 O3 + hν
O( D) + O2

1 . O( D) + H2O
2 OH

. Nitrate radical, NO3 is extremely important in night-time atmospheric chemistry. . . At night the NO3 can act as a hydrogen atom abstractor in the same way as OH It forms as :

10 NO2 + O3
NO3+ O2

NO3 has low day-time concentration because if photolysis ( λ < 633nm):

NO3+ hν
NO2+ O

3. Summary of basic gas-phase chemical reactions in the stratosphere.

NOTE: detailed discussion of stratospheric chemistry is given in Lectures 35-38.

Background stratospheric condition (no chlorine and other halogens): Ozone production: 1 O2 + hν
O( D) + O (λ < 175 nm) O(1D) + M
O + M

O2 + hν
O + O (175 nm < λ < 245 nm) then

O + O2 + M
O3 + M

NO catalytic ozone destruction:

NO + O3
NO2 + O2

NO + O
NO + O2

Net: O + O3
2 O2

OH. catalytic ozone destruction: . . OH + O3
HO2 + O2 . . HO2 + O
OH + O2

Net: O + O3
2 O2

11 OH. is produced in the stratosphere by one of the several mechanisms: 1 . O( D) + H2O
2 OH 1 . . O( D) + CH4
CH3 + OH 1 . . O( D) + H2
H + OH

Polluted stratospheric conditions: chlorine and bromine both affect ozone concentrations in the stratosphere.

Cl catalytic ozone destruction:

Cl + O3
ClO + O2

ClO + O
Cl + O2

Net: O + O3
2 O2

Br catalytic ozone destruction:

Br + O3
BrO + O2

BrO + O
Br + O2

Net: O + O3
2 O2

12